1. Field of the Invention
Embodiments of the invention generally relate to methods and systems for reducing pressure of natural gas and, in particular, to methods and systems for injection delivery of compressed natural gas.
2. Background
It is a well-known practice to compress non-ideal gases, including elemental and other gases for scientific or industrial purposes, for transport and delivery to consumers or other customers. For example, it is a known practice to transport compressed natural gas (CNG) by truck, ship, or similar delivery system to users that periodically require natural gas supply in excess of the supply available through existing pipelines. Further, there are areas in which natural gas service via pipeline is not available at all, due to remoteness, the high cost of laying pipelines, planned or unplanned outages, or other factors. In such cases, tanks of CNG transported by truck, for example, can be an economical way to provide the natural gas service required by such users.
To be economical, such tanks must be filled with large amounts of usable natural gas. Accordingly, full tanks of CNG are under very high pressure, commonly around 3000 pounds per square inch gauge (psig). However, in many cases natural gas under considerably lower pressure, e.g., from 20 psig to 100 psig, is required. Consequently, unloading a CNG tank requires a substantial reduction in the gas pressure prior to being received at a customer's intake. Currently, reducing the pressure of the CNG may be problematic due to substantial cooling of the natural gas caused by the Joules-Kelvin effect. Allowing a large volume of CNG to be depressurized results in a large temperature drop that can expose the material that comprises CNG tanks, valves, pipelines (particularly carbon steel pipes), customer equipment or other pieces of a natural gas system to low temperatures possibly exceeding safe operating ranges specified by manufacturers and codes.
Users of CNG supply systems may require volumes of natural gas that range from very low flow to flows in excess of 25,500 standard cubic feet per hour (scfh). At such rates, the cooling resulting from depressurization may be transmitted a significant distance downstream from the point of regulation. This may increase the chance of failure if the material or equipment at the customer's intake is not rated for the extreme cold temperature of the gas. Such failures could result in a loss of a substantial volume of gas through a relief valve that releases gas to the atmosphere when pressure is too high. At worst, a failure could result in irreparable damage or destruction of equipment and/or explosion.
It is understood that there are electric or electronic devices, control valves, and/or pressure controllers that may be able to accept the high-pressure CNG, depressurize it, and pass it to a standard natural gas intake at a relatively high rate of delivery. Such devices are extremely expensive, however, reducing or eliminating the profitability of truck-delivery of CNG. Further, devices capable of operating at the temperature ranges produced by extreme depressurization of natural gas are not readily available.
Accordingly, there is a need in the industry for a reliable natural gas delivery system that provides depressurized gas at a steady rate with varying flow conditions.